Computer Networks are used to interconnect computers and peripherals to allow them to exchange and share data such as files, electronic mail, databases, multimedia/video, and other data.
Packet Switching
Nearly all computer networks use packet switching, which divides longer units of data transfer into smaller packets which are sent separately over the network. This allows each packet to be processed independently from another packet without having to wait for the entire data transfer to be completed. It also enables communications between a plurality of computer systems to be intermixed on one network. Host interfaces connect the computer systems to a network allowing each computer system to act as the source and destination of packets on the network.
A first key issue in packet switched networks is addressing. The addressing in packet switched networks is conventionally performed by one of two approaches, known as virtual circuit packet switching or datagram, packet switching.
Virtual Circuit Packet Switching
In the virtual circuit approach, before any data can be transmitted, a virtual circuit must be first established along the path from the source to the destination in advance of any communication. After the virtual circuit is setup, the source can then send packets to the destination. Each packet in the virtual circuit approach has a virtual circuit identifier, which is used to switch the packet along the path from source to the destination.
The virtual circuit approach reduces the size of the identification required in each packet header. It also allows additional information about the packet handling to be established as part of the virtual circuit setup operation. Another claimed benefit is that forwarding and switching of virtual circuit packets can be made more efficient because of the virtual setup process. However, the virtual circuit approach incurs the cost of delay to setup the virtual circuit before sending any data, and it incurs the cost of maintaining the virtual circuit state in each network device along the virtual circuit path, even if a virtual circuit is idle. Also, in practice the memory space for virtual circuit state in network devices has limited the number of circuits that are available, which complicates the behavior of network nodes that need to create virtual circuits to communicate.
Datagram Packet Switching
In the datagram approach, each datagram packet is a self-contained unit of data delivery. A typical datagram packet includes a globally unique source address, a globally unique destination address, a protocol type field, a data field, and a cyclic redundancy checksum (“CRC”) to insure data integrity.
Datagrams can be sent without prior arrangement with the network, i.e. without setting up a virtual circuit or connection. Each network device receiving a datagram packet examines the destination address included in the datagram packet and makes a local decision whether to accept, ignore, or forward this packet.
Various conventional network devices learn information from observing datagram packet traffic in data networks. For example, a conventional network switch device that interconnects multiple network segments can “learn” the location of network stations connected to its ports by monitoring the source address of packets received on its ports. After it has associated a station address with a certain port, the network switch can then forward datagram packets addressed to that station to that port. In this type of device, the datagram source address is used to learn the location of a station on the network, whereas the forwarding decision is made on basis of the datagram destination address alone.
Datagram packet switching has the advantage that it avoids the overhead and cost of setting up virtual circuit connection in network devices. However, it incurs the expense of transmitting a larger packet header than required for virtual circuit switching, and it incurs the cost for processing this larger packet header in every network device to which it is delivered. Also, there is no virtual circuit setup process to establish additional information for datagram packet processing. Another disadvantage of datagram packet switching is that it is difficult to control packet flow to the same degree as with virtual circuits because there is, in the conventional case, no state in the network devices associated with the traffic flow.
The datagram packet switching approach has been extensively used in shared media local area networks. Shared media networks provide for a multiplicity of stations directly connected to the network, with the shared media providing direct access from any transmitter to any receiver. Since the receivers need to be able to distinguish packets addressed specifically to them, each receiver needs to have a unique address. In addition, since the unit of access to the shared medium is one packet, each packet needs to contain the unique address capable to identify the receiver. As a result, all commonly used local area networks are based on datagram packet switching and have no provisions for virtual circuit setup.
Media Access Control Protocol
The network access mechanism in shared media local area network will now be further described. This function, commonly known as the media access control or MAC protocol, defines how to arbitrate access among multiple stations that desire to use the network. Individual stations connected to the network have to adhere to the MAC protocol in order to allow proper network operation.
A number of different media access control protocols exist. The MAC protocol, in conjunction with the exact packet format, is the essence of what defines a local area network standard. The following is a brief overview of local area network standards that are in wide use today.
The most widely used local area network is commonly known as Ethernet and employs an access protocol referred to as Carrier Sense Multiple Access with collision Detection (CSMA/CD). [see U.S. Pat. No. 4,063,220, issued Dec. 13, 1977, for a Multipoint Data Communication System with Collision Detection, Inventors Metcalfe, Boggs, Thacker, and Lampson]. The current definition of the Ethernet CSMA/CD protocol is defined in IEEE Standard 802.3, published by the Institute of Electrical and Electronics Engineers, 345 East 45th Street, New York, N.Y. 10017. The Ethernet standard specifies a data transmission rate of 10 or 100 Megabits/second.
Another widely used local area network standard is Tokenring, also known as IEEE Std 802.5, transmitting at a speed of 4 or 16 Mbits/sec and FDDI or Fiber-Distributed-Data-Interface which sends data at a speed of 100 Mbits/sec. Both Tokenring and FDDI are based on a circulating token granting access to the network, although their respective datagram packet formats and other operating aspects are unique to each standard.
What is common to all these media access control mechanisms is that they do not include provisions for virtual circuit setup and have no provisions to specify attributes that relate to virtual circuits, such as traffic management or flow control for specific connections. This limits the ability of conventional local area networks to accommodate higher level network functions or to support virtual connection oriented traffic mechanisms.
Devices for Interconnecting Local Area Networks
Another key issue with datagram packet switched networks is how to interconnect individual network segments into larger networks. The size and usage of datagram packet switched networks has grown much beyond what was envisioned when these networks were designed. Devices such as bridges, switches, and routers, have been used to interconnect individual LAN segments into larger networks, but each have their own set of problems in scaling to higher performance.
Bridges forward datagram packets between network segments by learning the location of the devices on the network by observing the source address contained in datagram packets passing by. Once the bridge has learned which network device is located on which network segment, it can then forward datagram packets addressed to that network device to the appropriate network segment. One of the limitations of bridges is that they do not filter traffic beyond the data link level.
Switches are basically multi-port network bridges that can forward datagram packets among a multiplicity of network ports. Frequently, switches provide additional capabilities for assisting with network management, including traffic filtering and segmenting networks into virtual LANs. As in the case of bridges, switches have to forward broadcasts to all ports configured into one virtual LAN. In addition, conventional switches cannot provide fair service or priority service to individual traffic flows, and they require significant amount of memory to avoid dropping packets in the case of network congestion.
Routers also interconnect several network segments, but they operate primarily at the network protocol layer, rather than at the datagram packet layer. Routers participate in all network protocol functions, which traditionally requires general purpose processing. As a result, traditional routers are more expensive and have less throughput than switches. In addition they are more difficult to administer.
Finally, virtual circuit packet switched networks, in particular ATM, have been proposed to interconnect local area network segments. However, it has turned out to be very difficult to map existing network protocols that are based on datagram packets to the ATM network architecture.
In summary, bridges and switches transparently extend the domain of networks, and allow for cost-effective and high-performance implementations. However, they cannot segment a network effectively in terms of traffic management and broadcast traffic. Routers, on the other hand, can segment networks very effectively, but are much more expensive and are performance bottleneck in high-speed networks. ATM has been very difficult to map to current network protocols.
The ideal network device for interconnecting network segments would have the high-speed and cost-effectiveness of a switch, with the ability of segment and manage network traffic similar to a router.
Traffic Management
Another key issue in packet switched networks is traffic control or traffic management.
In a packet switched network, each link at every switching node in the network represents a queue. As the traffic arrival rate at a link approaches its transmission rate, the queue length grows dramatically and the data in the queue needs to be stored in the attached network nodes. Eventually, a network node will run out of packet buffer capacity which will cause further packets arriving to be discarded or dropped. Dropped packets are eventually retransmitted by the source, causing the traffic load to increase further. Eventually, the network can reach a state where most of the packets in the network are retransmissions.
Conventionally, two types of traffic control mechanism are used in packet switched networks: flow control and congestion control. Flow control is concerned with matching the transmission rate of a source station to the reception rate of a destination station. A typical networks flow control mechanism uses a window techniques to limit the number of packets a source can transmit which are not yet confirmed as having been received by the destination. Conventional flow control is an end-to-end mechanism that exists in certain network protocols, in particular connection oriented network protocols such as TCP/IP. However, conventional flow control between source and destination does not solve the network congestion problem, since it does not take the utilization of buffer resources within the network into account. In addition, non-connection oriented network protocols do not use window based flow control. Also, continuous rate traffic sources such as real-time video don't match the nature of destination controlled behavior since the transmission rate is determined by the source.
Problem Statement
What is needed is an improved method and apparatus for high-speed datagram packet switched networks that can support a large number of network stations, a wide range of network transmission speeds, a wide variety of source traffic behavior including video and multimedia, while maintaining compatibility with existing network protocols and applications.